Electronic devices, such as palm computers, digital cameras and cellular telephones, are becoming more compact and miniature, even as they incorporate more sophisticated data processing and storage circuitry. Moreover, types of digital communication other than text are becoming much more common, such as video, audio and graphics, requiring massive amounts of data to convey the complex information inherent therein. These developments have created an enormous demand for new storage technologies that are capable of handling more complex data at a lower cost and in a much more compact package. Efforts are now underway to enable the storage of data on a scale of ten nanometers (100 angstroms) up to hundreds of nanometers, referred to herein as “ultra-high density data storage.”
One method of storing data at ultra-high densities involves utilizing a directed energy beam. As used herein a “directed energy beam” means a beam of particles, such as electrons, or a beam of photons or other electromagnetic energy, to heat the [GG1]medium so that it changes states[GG2][GG3]. As used herein, “state” is defined broadly to include any type of physical change of a material, whether from one form to another, such as crystalline to amorphous, or from one structure or phase to another, such as different crystalline structures. As used herein, the term “phase change” means a change between different states in a material.
A state change may be accomplished by changing a material from crystalline to amorphous, or the reverse, by the application of an electron or light beam. To change from the amorphous to crystalline state, beam power density is increased so as to locally heat the medium to a crystallization temperature [GG4]. The beam is left on long enough to allow the medium to anneal into its crystalline state. To change from crystalline to amorphous state, the beam power density is increased to a level high enough to locally melt the medium and then rapidly decreased so as to allow the medium to cool before it can reanneal. To read from the storage medium, a lower-energy beam is directed to the storage area to cause activity, such as current flow representative of the state of the storage area.
An example of an ultra-high density storage device is given in U.S. Pat. No. 5,557,596 granted to Gibson et al. on Sep. 17, 1996 (“Gibson”). In Gibson, a plurality of electron emitters direct beams of electrons to a phase-change layer in data storage media. The electron beams are used to write data by causing a change of state in the phase-change layer, and read data by emitting lower energy beams to generate activity at the local storage areas indicative of the state of each storage area.
To effectively sense contrasts in states or phases of phase-change materials, a diode may formed having a junction for sensing carrier flow in response to an electron or light beam focused on a data storage memory cell in the phase-change layer. Such diode junctions are utilized for carrier detection in photovoltaic devices, in which light beams impact the diode, and in cathodovoltaic devices, in which electron beams are directed to the diode. Photovoltaic devices include phototransistor devices and photodiode devices. Cathodovoltaic devices include cathodotransistor devices and cathododiode devices. In addition, diode junctions may be utilized for carrier flow detection in photoluminescent and cathodoluminescent devices. Reference is made to copending patent application Ser. No. 10/286,010, filed on Oct. 31, 2002 for a further description of the structure and function of diode junctions in these devices.
Diode junction layers need to be composed of materials having electrical properties suitable for generating a desired carrier flow across the diode junction. As used herein, the term “carrier flow” refers to either electron current or the flow of holes, depending on whether the materials are n-type or p-type. As used herein, the term “materials” includes all kinds and types of compounds, alloys and other combinations of elements. Various types of junctions may be formed in the context of the above data storage devices, such as heterojunctions, homojunctions, and Schottky junctions, in order to achieve the desired detection results.
Junction problems can sometimes be avoided by forming a homojunction using the same material for both layers of the diode. However, in such case, it is usually necessary to dope one or both layers, in order to form a suitable diode having one layer with p-type characteristics and the other layer with n-type characteristics. Some materials do not readily accept doping. Furthermore, doping usually increases the fabrication steps needed and the complexity of fabrication to form the diode layers. It may also be desirable to dope the semiconductor materials used in heterojunction and Schottky diodes if the carrier density and/or resistivity of the material need to be adjusted.
In some cases, one of the diode layers may also function as the phase-change layer of the data storage device. One material being used for a phase-change layer in such data storage devices is an indium chalcogenide compound, such as indium selenide (InSe). The material has suitable phase change characteristics in transitioning between first and second phases, where both phases exhibit different electrical properties. However, there are difficulties with InSe forming a suitable diode layer. Although single crystal InSe material may be doped, polycrystalline InSe is naturally an n-type material and cannot be readily p-doped. Thus, using polycrystalline InSe as a phase-change layer limits the choices of a suitable second layer with which it can form a data detecting diode junction. In that case, the second layer usually must have p-type electrical properties to form a suitable carrier flow across a diode junction.